This article studies the ontology proposed by Xavier Zubiri in his
work Sobre la esencia [On Essence], in order to discover how he
treats the problem of the quidditative essence. After certain prior
considerations, the article examines the traditional conception of
specific essence, gradually disarticulating the essence-speciesindividual
relation. With the phylum understood as a constitutive
scheme transmitted genetically, and the species as that which
constitutes the belonging of an organism to its phylum, the
quidditative essence is shown to be a moment of the individual
constitutive essence.